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CoolCampanula

VernalizationSpecies Requirement Hardiness Zone Notes

C. carpatica None 3 Plants can be held at 41° F for up to 15 weeks, the longest duration tested.

C. garganica Obligate 5 No plants flowered without cold in our trials.

C. glomerata Facultative 2 Less than 20 percent of starting material with 4-9 leaves flowered without vernalization, more than 80 percent with. Cold treatment had little effect on time to flower.

C. persicifolia Obligate 3 Some persicifolia cultivars reportedly require bulking before vernalization; vegetatively propagated ‘Chettle Charm’ did not. We tested vernalization for 0, 6 or 12 weeks, and only 12 weeks flowered completely.

C. portenschlagiana Obligate 4 No plants flowered without cold in our trials.

C. poscharskyana Facultative 3 Cooling increased flowering percent and bud number and decreased time to flower by approximately 20 days. Supplemental lighting increased bud number in uncooled plants.

C. punctata Obligate 6 Cooling is essential for complete and consistent flowering. Complete flowering was achieved by bulking ‘Cherry Bells’ for 15 weeks and vernalizing for 10 weeks at 41° F. ‘Wedding Bells’ did not flower without cooling; less than half flowered with cooling.

Note: Recommendations are based on the specific cultivars tested. These recommendations may not be applicable to other cultivars within the same species, though they should make a goodstarting point for experimentation. Unless stated otherwise, plants were vernalized in plug trays at 41° F for 15 weeks.

Figure 1. Vernalization requirements of select campanula species.

Cooling and daylength can regulate flowering of some campanula species and cultivars.

By Sonali Padhye, Cathy Whitman, Erik Runkle and Art Cameron

Top: Campanula punctata ‘Cherry Bells’ forced at 68° F without vernalization treatment did not flowerunder (from left to right) 9-hour photoperiod, 16-hour photoperiod provided by incandescent lamps and16-hour photoperiod provided by high-pressure sodium lamps. Bottom: Campanula punctata ‘CherryBells’ forced at 68° F following 15-week vernalization treatment at 41° F (from left to right) did not flowerunder 9-hour photoperiod but did flower under 16-hour long days provided by incandescent lamps andhigh-pressure sodium lamps.

The genus campanula contains more than 300 species, many ofwhich are of Northern origins. Campanula species are com-monly referred to as bellflowers and can add a great splash ofblue, white or red to any perennial garden.

Many campanulas such as C. carpatica, C. portenschlagiana and C.poscharskyana thrive in cool temperatures and high light conditions, makingthem very suitable for the springs and summers in Northern Europe.Therefore, it is not surprising that in Northern Europe campanulas areextremely popular and almost a staple in any perennial garden. Severalcampanulas, especially many noteworthy cultivars such as C. punctata

‘Cherry Bells’ and campan-ula ‘Kent Belle’ are report-edly adaptable to high heatand humidity. A few cam-panulas, including C. rotun-difolia, are native to theUnited States and offergreat untapped potential togardeners and plant breed-ers looking for new plantmaterial. Smaller-sizedcampanulas such as C.carpatica form charmingmounds of flowers in smallcontainers and are impor-tant potted flowering crops.Several campanulas suit-able for domestic condi-tions have been significantto the American herbaceousperennial market. ÁCampanula persicifolia ‘Chettle Charm’

Long days with Short days Long days supplemental light

Flowering campanulasWhen perennials are offered in flower, their marketability is boosted

considerably. Forcing perennials and scheduling them to flower on a spe-cific date requires an understanding of perennial flower induction mecha-nisms. Our research program has studied several campanulas, and it isintriguing to review unique strategies to force them.

Flowering or sexual reproduction is considered the culmination of a plant’sdevelopmental process. In order to ensure reproductive success, plants have

evolved highly regulated mechanisms that operate on environmental cues.Thus, plants flower when the environment is most conducive for floweringand pollination. Two important environmental signals that plants, especiallyin Northern origins, respond to are exposure to cool temperatures, known asvernalization, and response to daylength, known as photoperiodism.

Their similar botanical characteristics cause campanulas to be groupedin the same genus, but we have found that campanula species and culti-vars vary greatly in their flowering requirements. We will overview theflowering requirements of campanulas that enable these plants to beforced into bloom for subsequent market sales.

Vernalization RequirementsWe have found that campanula species vary widely in their vernalization

requirements (see Figure 1, page 72). Campanula carpatica has no vernaliza-tion requirement. In our studies, plants that were not cooled and plants thatwere cooled for short or long durations were similar in their flowering per-cent, time to flower and other flowering characteristics such as bud numberand height at flowering. It is interesting that C. carpatica is very cold tolerantand can be held at low temperatures for extended periods, but exposure tocold is not required for flower induction.

Campanula glomerata and C. poscharskyana showed a facultative or quantitativevernalization response. During our research only a few plants flowered with-out cooling, but after cooling for 15 weeks at 41° F flowering percent increasedsignificantly. Although cooling did not affect the flowering time of C.glomerata, C. poscharskyana flowered approximately three weeks sooner follow-ing vernalization compared to non-vernalized plants.

Campanula garganica, C. persicifolia, C. portenschlagiana and C. punctata havean obligate or qualitative vernalization requirement. These species did not flower unless cold was provided; however, vernalization induced thesespecies to flower. It is essential to overcome the juvenile phase before Á

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‘Birch Hybrid’ were vernalized for (from left to right) 0, 3 or 6 weeks and then forced in thegreenhouse at 68° F. No plants flowered without vernalization; all flowered after three weeks ofvernalization. Flower number and time to flower were significantly improved after six weeks ofvernalization. (Photo courtsey of Leslie Finical)

Vernalization Hybrid Requirement Notes

‘Birch Hybrid’ Obligate Cross of C. portenschlagiana and C. poscharskyana. Plants were vernalized for 0, 3, 5, 7, 9 or 12 weeks. All plants flowered with at least 5-weeks cold treatment. Time to flower decreased by two weeks when vernalization increased from five to seven weeks with no further effect from additional vernalization.

‘Kent Belle’ Facultative Reportedly derived from C. punctata, C. latifolia and C. takesimana. Only 10-40 percent of plants flowered without vernalization,100 percent after vernalization.

Figure 2. Vernalization requirements of select campanula hybrids. Unless stated otherwise, plants were vernalized in plug trays at 41° F for 15 weeks.

beginning the cooling period. Some C.persicifolia cultivars reportedly requirebulking, but in our preliminary screenthe vegetatively propagated cultivar‘Chettle Charm’ flowered readily with-out bulking.

‘Kent Belle’ reportedly has C. punc-tata, C. latifolia and C. takesimana in itsparentage. In our studies, ‘Kent Belle’has a facultative vernalization require-ment. Percent flowering of non-ver-nalized ‘Kent Belle’ plants rangedfrom 10 to 40 percent depending onthe light regime provided, but following ver-nalization all plants bloomed completely,regardless of the light regime tested. ‘BirchHybrid’ is reportedly a hybrid between C.portenschlagiana, having obligate vernalizationrequirement, and C. poscharskyana, having fac-ultative vernalization requirement. ‘BirchHybrid’ has “inherited” an obligate vernaliza-tion requirement. We have maintained stockplants of ‘Birch Hybrid’ for the last 2-3 years. Inrare instances, overgrown stock plants initiate afew flowers; however, flowering is non-uni-form and very sparse. Therefore, horticulturallyspeaking, ‘Birch Hybrid’ requires vernalizationfor flowering.

Vernalization and HardinessThe observed diverse vernalization responses

prompted us to explore whether there is an obvi-ous link between vernalization requirements anda plant’s native range or reported USDA ColdHardiness Zone. Campanula carpatica, which is ofvery Northern origin and cold hardy to Zone 3,has no vernalization requirement; whereas, C.glomerata and C. poscharskyana, which are alsonative to the North and cold hardy to Zones 2

and 3, respectively, have a facultative vernaliza-tion requirement (see Figure 1, page 72).

Finally, C. garganica, C. persicifolia, C. portenschla-giana and C. punctata have obligate vernalizationrequirements yet varying cold hardiness betweenZones 3 and 6. Thus, for campanula there does notappear to be a direct relationship between coldhardiness and vernalization requirement.

variety information

Photoperiodic Hybrid Requirement Notes

‘Birch Hybrid’ Day neutral Complete flowering after sufficient vernalization. Plants were more compact under SD. Supplemental lighting considerably increased the number of buds.

‘Kent Belle’ Day neutral Photoperiod had little effect on time to flower, bud number or plant height at flowering.

Figure 4. Photoperiodic requirements of select campanula hybrids.

Photoperiodic Species Requirement Notes

C. carpatica Obligate LD The critical photoperiod of ‘Blue Clips’ is 14 hours. Sixty percent of plants flowered between 13 and 14 hours; all plants flowered at more than 16 hours. If LD are provided as night interruption to ‘Blue Clips’, lighting should be for four hours in the middle of the night, and lights should be on constantly.

C. garganica Facultative LD LD increased percent flowering, hastened flowering by approximately 25 days and considerably increased bud number.

C. glomerata Facultative LD LD increased flowering percent and hastened flowering by approximately 20 daysbut did not consistently affect bud number.

C. persicifolia Day neutral Plants flowered under all photoperiods.

C. portenschlagiana Day neutral Photoperiod did not affect flowering characteristics.

C. poscharskyana Facultative LD LD hastened flowering by approximately 20 days and increased bud number and plant height at flowering.

C. punctata Obligate LD After sufficient vernalization, no ‘Cherry Bells’ flowered under SD in 15 weeks; all flowered under LD. After 15-week vernalization, no ‘Wedding Bells’ flowered under SD; complete flowering did not occur under LD.

Figure 3. Photoperiodic requirements of select campanula species. Long-day (LD) and short-day (SD) plants were grown under nine hoursof natural light provided by blackout system from 8 a.m. to 5 p.m. with supplemental high-pressure sodium lighting when the light levelswere below 140 µmol·m-2·s-1. LD plants received an additional seven hours of lighting by incandescent lamps provided from 5 p.m to 12 a.m.

Top: ‘Birch Hybrid’ forced under (from left to right) 63, 68,73, 79 and 84° F. Note the reduced plant size and flowernumber under higher temperatures. Bottom: ‘Birch Hybrid’forced under (from left to right) 63, 68, 73, 79 and 84° F. Notethe reduced size and faded color under higher temperatures.(Photos courtesy of Alison Frane)

When trying to identify the forcing require-ments of new campanula species and cultivars,new plants should be tested under controlledconditions since there have been no obviousrelationships between a species’ native rangeand its vernalization response type. Althoughthe vernalization response types of hybridstend to be similar to that of a parent, withoutconducting experiments it would be impossi-ble to guess whether a hybrid would have avernalization requirement or not.

Temperature and DurationMost of our vernalization recommenda-

tions are based on cooling plants at 41° F.Our research results indicate that ‘BirchHybrid’ flowers in response to a very broadrange of temperatures — 32-55° F, depend-ing on the duration of exposure. Note thatvernalization at 55° F only delayed flowering1-2 weeks compared to the quickest flower-ing treatment, which was 41° F. Completeflowering of ‘Birch Hybrid’ was achieved Á

variety information

C. carpatica 0 LD: greater 7-8 5-7 Bulk plants to desired size under SD prior to LD. Supplemental ‘Blue Clips’ than or at lighting increased bud number. Larger flowers and more intense

16 hours color when grown cooler.

C. carpatica 0 LD 8 6-7 As uniform as other cultivars. Supplemental light increased‘Blue Uniform’ number of buds.

C. carpatica 0 LD 8 5-6 Supplemental light increased bud number. Very similar ‘Deep Blue Clips’ to ‘Blue Clips’.

C. carpatica 0 LD 7 5-6 Supplemental light increased bud number.‘Experimental Blue’

C. carpatica 0 LD 7-8 5-7 Supplemental light increased bud number. Culture similar to ‘White Clips’ ‘Blue Clips’.

C. garganica 15* LD 7 5-6 Plants may have juvenility, and bulking may be essential prior to vernalization.

C. glomerata 15* LD 6 14-16 No treatment gave 100-percent flowering when starting material had 4-9 leaves. Juvenility may be a problem. Little response to supplemental lighting.

C. portenschlagiana 15* Natural 5 3-5 Plants continue to flower over a long period of time.

C. poscharskyana 15* LD 6 13-14 Supplemental lighting increased flowering in non-vernalized plants but did not affect bud number.

C. persicifolia 12 LD** 6-7 15-24 Vegetatively propagated with no juvenility. Suitable for 1-gal. or larger ‘Chettle Charm’ or more containers. PGRs may be required. May become aggressive.

C. punctata 10 LD 5-6 15-18 Supplemental lighting resulted in 100-percent increase in bud number. ‘Cherry Bells’ or more Plant height and flower size were decreased at higher temperatures,

but all plants were acceptable quality. Suitable for 1-gal. or larger containers. PGRs may be required. May become aggressive.

C. punctata 15* LD 7-8 12-18 May have a juvenility and should be bulked before cooling. ‘Wedding Bells’ Supplemental lighting increased bud number.

‘Birch Hybrid’ 5 Natural 4-5 5-7 Supplemental lighting increased bud number. Flower size and color or more improve under cool forcing temperature.

‘Kent Belle’ 15* Natural 7-8 15-25 Suitable for 1-gal. or larger containers. PGRs may be required.

*Only 0- and 15-week vernalization were tested.**LD was the only photoperiod tested.

Figure 5. Forcing recommendations for campanula species and hybrids. Long-day (LD) plants were grown under nine hours of naturallight provided by blackout system from 8 a.m. to 5 p.m. with supplemental high-pressure sodium lighting when the light levels were below140 µmol·m-2·s-1 and additional seven hours of lighting by incandescent lamps 5 p.m. to 12 a.m. All plants were vernalized in plug trays at41° F under 9-hour photoperiod provided by fluorescent lamps. No plant growth regulator applications were made.

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‘Kent Belle’ flowered when forced at 68° F following vernalization at 41° Ffor 15 weeks under (from left to right) 9-hour photoperiod and 16-hourphotoperiod provided by incandescent and high-pressure sodium lamps.Plants under 9-hour photoperiod flowered later than plants under 16-hourphotoperiod. (Photo courtesy of Art Cameron)

after five weeks of vernalization at 32-46° F, and plants vernalized at 32 and37° F flowered approximately 10 days later than those at 41 and 46° F.

When we moved actively growing 4-week-old ‘Birch Hybrid’ plugs from68 to 28° F the plugs often died during vernalization. Since ‘Birch Hybrid’ isvery cold tolerant in the garden and is cold hardy to Zone 4 (-30 to -20° F), webelieve exposing actively growing plant material to 28° F does not allow it toharden, resulting in plant death. Therefore, growers who wish to vernalizeplants without pre-hardening should avoid vernalizing at or less than 28 °F.

PhotoperiodismWe have also found that the photoperiodic requirements for flowering

Campanula species vary greatly (see Figure 3, page 76). Campanula persici-folia and C. portenschlagiana are day neutral and, therefore, flower underany photoperiod. Also, the photoperiod does not affect flowering charac-teristics such as plant form, height, flower number, etc.

Campanula garganica, C. glomerata and C. poscharskyana are facultative long-day plants. Under long days, C. garganica and C. glomerata flowered com-pletely and faster, and C. garganica also had more buds. On the other hand,C. poscharskyana flowered under short and long days but flowered fasterunder and had more buds when forced under long days.

Campanula carpatica and C. punctata are obligate long-day plants. Whengrown under short days, these plants form rosettes; when grown under longdays, plants elongate and bolt during flowering. The critical photoperiod ofC. carpatica ‘Blue Clips’ is 14 hours. Therefore, at any photoperiod less than14 hours plants form rosettes, and at or greater than 14 hours plants elongateand flower. It should be noted that ‘Blue Clips’ flowers faster under photope-riods of 16 or more hours rather than 14 hours.

Both ‘Birch Hybrid’ and ‘Kent Belle’ are day-neutral plants and can beforced under any photoperiod (see Figure 4, page 76).

For the different campanula species tested in our program, we did notobserve any apparent relationship between the native ranges, cold hardi-ness, vernalization requirements and photoperiodic requirements for flow-ering. Thus, along with vernalization requirements, the photoperiodicrequirements for flowering campanulas must be tested in controlled experi-ments to ensure successful forcing.

Forcing CampanulasCampanulas can effectively be forced into flower by manipulating their pho-

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‘Blue Clips’ were forced under (from left to right) 10-, 12-, 13-, 14-, 16- and 24-hour photoperiod and 9-hour photoperiod with 4-hour night interruption provided by incandescent lamps. Sixty percent ofplants flowered under 14-hour photoperiod, and all plants flowered under photoperiods of greater thanor at 16 hours. Also, flowering under 14-hour photoperiod was slower. (Photo courtesy of Art Cameron)

toperiod, vernalization requirementor both (see Figure 5, page 77). Alongwith the forcing requirement, culturalrequirements should also be consid-ered prior to campanula production.

Growing temperature. The prima-ry effect of forcing temperature is theregulation of plant growth rate. Asthe forcing temperature increasestime to flower decreases. However,forcing temperature also has sec-ondary effects on flowering charac-teristics. Growing campanulas underhigh temperatures not only inducesthem in flower quickly but alsoreduces plant height, decreasesflower number and size, and reducesthe intensity of flower color. As men-tioned earlier, most campanulas pre-fer cool seasons and should be forcedunder cool temperatures to obtainmore aesthetically appealing plants.We have produced campanulas ofgood quality under 68° F in ourresearch program.

Light quantity. Most campanu-las respond favorably to lightquantity, regardless of their pho-toperiodic requirements. The mostcommonly observed effect of sup-plemental lighting is increased budnumber, as reported in several cul-tivars including all carpatica culti-vars tested and ‘Birch Hybrid’. Theonly exception to this was C. glom-erata, which showed no responseto light quantity. Often times,adding supplemental lighting alsohastens flowering. However, sup-plemental lighting increases planttemperature, and therefore, quick-er flowering may be due to theincrease in plant temperature andnot the light quantity.

Plant growth regulators.Typically, plant growth regulatorapplications are effective at reduc-ing the height of the campanulaswe tested, though under ourexperimental conditions, the actualresponse of different cultivars var-ied. For example, A-Rest (SePRO),B-Nine (Chemtura Corporation),Bonzi (Syngenta ProfessionalProducts), Cycocel (OHP) andSumagic (Valent USA) were alleffective in height control of C.carpatica. Whereas, ‘Kent Belle’responded well only to A-Rest, B-Nine, Cycocel and Sumagic. Therates, timing and choice of chemi-cal depend on growing conditionsand should be tested prior to use.

SummaryIn summary, there are many cam-

panulas suitable for production aspotted crops. Scheduling campanu-las is easier when manipulating pho-toperiod, vernalization requirements

or both. Most campanulas testedfavored high light conditions andresponded well to most plantgrowth regulators tested.

Sonali Padhye is a PhD student,Cathy Whitman is a research tech-nician, Erik Runkle is assistant pro-

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fessor and floriculture extensionspecialist, and Art Cameron is pro-fessor in the Department ofHorticulture at Michigan StateUniversity, East Lansing, Mich.Cameron can be reached by phoneat (517) 355-5191 x338 or E-mail atcameron@msu.edu.

Authors’ Note: We wish to thank thesupporters of the Michigan State UniversityFloriculture Research Program.

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